Diverse forms of construction cranes are known including mobile tower and truss cranes, some of which are equipped with luffing jibs. Such cranes have a notable deficiency in that the length of the tower or radius of the truss boom is fixed. This reduces the operating versatility of these cranes. So-called hammerhead cranes are widely used in large building construction and possess the advantage of being able to service the entire horizontal area spanned by the building by means of the load carriage which traverses the horizontal counterweighted crane boom. The disadvantage of the hammerhead crane is that it is erected with the building and is dismantled only after completion of the building construction. It therefore lacks mobility.
A third commonly used crane is the mobile hydraulically operated telescoping multi-section boom crane with full luffing and sluing capability. This latter type of crane, while exceedingly versatile, lacks some of the capabilities of larger tower and truss types, as well as hammerhead cranes, for placing or retrieving loads at or from central locations on tall structures where the loads are quite distant from the structure side walls. The luffing jib structure forming the main subject matter of this invention is employed with the variable radius mobile telescoping boom crane, discussed immediately above.
The main objective of the invention is to fulfill a particular need in the art for a highly mobile variable radius crane which will be capable of doing much of the work which presently can only be done by larger truss and tower cranes or the completely immobile hammerhead types. More particularly, the present invention provides a large luffing jib for mounting atop the telescoping boom of a mobile hydraulically operated crane to greatly expand the utility and the versatility of such a crane.
Another object is to provide a luffing jib assembly which is easy to erect and which folds or collapses into a very compact form for ready transport without separating the component parts thereof. Since the parts are permanently connected with each other and with the jib proper, there is no possibility of the parts becoming lost in transit or misassociated in any way. Set-up time and dismantling of the luffing jib is also rendered less time-consuming.
A further object is to provide in a luffing jib structure for crane booms an assemblage of pivotally connected masts and cooperative folding ladders forming a part of the pendant line between the point of the boom and the bridle assembly which possesses an automatic operating mode when the jib is swung from a near vertical to a near horizontal position, in terms of transferring the pendant line tension load to the third mast and preventing the taut pendant line above the bridle from moving too close to the rear of the crane boom.
A further object is to provide in a luffing jib for crane booms a system of three cooperating masts which are permanently pivotally connected to the base of the jib. The third mast of the system is controlled by a foldable tension link which interconnects the second and third masts and when fully extended during lowering of the jib toward a horizontal attitude lifts and rotates the third mast so that the latter will engage a gooseneck portion of a lower ladder connected in the pendant line and moving toward the nose of the boom. The engagement of the third mast with the gooseneck portion terminates the movement of the lower ladder toward the nose of the crane boom and precludes any fouling of the pendant line as the luffing jib approaches a horizontal position.
Still another important object of the invention is to provide a crane boom luffing jib which can be operated freely through a positive angle of 72 degrees above horizontal.
A further and more general objective is to provide a luffing jib structure which is simple, reliable in operation, strong and entirely practical.
In accordance with the present invention, there is also provided a means for storing the bridle assembly in the pendant line on one side of the crane boom and for conveniently moving the bridle assembly to a position at the top of the boom through a combined swinging and linear movement so that the bridle assembly will be in a proper position for release and use in conjunction with the luffing jib.
Other objects and advantages of the invention will become apparent during the course of the following description.